It was the latest in a spate of incidents in which threats or rumors posted online or distributed via social-networking sites spread fast to disrupt Midwest schools.

The Internet "makes it easier for unskilled vandals to hide behind anonymity," said Jon Giffin, an assistant professor of the School of Computer Science at the Georgia Institute of Technology in Atlanta.

"It's just the modern equivalent of someone calling in a bomb scare," Giffin said. The schools "have to take it seriously just in case it is an actual threat."

Public schools in Minneapolis went into a district-wide lockdown Wednesday because of a vague Internet threat that police traced to an IP address in Australia. The district kept its more than 33,000 students indoors at about 60 schools and all exterior doors locked. The "code yellow" lockdown continued Thursday but administrators lifted it around midday after police said the threat was not credible.

Police spokesman Sgt. Jesse Garcia said an e-mail was sent to the city's 311 communications center about 7:30 a.m. Wednesday warning that a posting on a social-networking site said a male would be coming to an unspecified school to "shoot up" the place then kill himself. The warning came from someone who had seen the post.

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Parents received a telephone alert message to notify them of the situation.

Cretin Derham-Hall High School in St. Paul was locked down Wednesday after a similar vague threat that also may have originated in Australia. The lockdown was lifted at the beginning of the school day Thursday.

Cyber threats are relatively new, said Mike Siitari, director of the Minnesota School Safety Center, part of the Department of Public Safety. He said his agency plans to start better tracking of online school violence threats.

In suburban Inver Grove Heights, Simley High School was forced to deal with online rumors that a shooting would happen Feb. 5 during a pep fest. The rumors proved unfounded, but Superintendent Deirdre Wells said "the level of anxiety among students was pretty high," so the pep fest was canceled.

"We have a responsibility to take things seriously, whether it's written on a bathroom wall or posted on somebody's Facebook page," Wells said Thursday.

Students don't understand that their Internet postings are not private, Wells said. "Even if they withdraw (a posting) it remains out there. You just can't take it off the Internet like it never happened."

In Farmington, high school principal Ben Kusch said "the rumor mill kicked in" on Facebook that "something bad was going to happen" after two small fights broke out at his school on Feb. 18.

"It came back to this very small group of students that had said some very foolish things, had posted some very foolish things without thinking on an online social-networking site," Kusch said.

Kusch said many students and parents were nervous, so the school brought in a couple of extra police officers as "a calming presence."

In central Nebraska, police officers carrying assault rifles were posted at Grand Island High School as a precaution after rumors spread about gang violence last month. The rumors started after a 15-year-old student was shot and wounded at a home and the suspect was still at large.

Minneapolis parents described some confusing moments on Wednesday as word spread about the lockdown.

Dawn Cassidy, whose daughter is a senior at Southwest High School, was at work and learned via a text from her daughter. She didn't hear the phone message from the school district until she got home Wednesday night.

"It's a scary thing to hear your kids' school is in a lockdown," said Cassidy, 50.

District officials were unable to say how the lockdown affected attendance. Spokeswoman Emily Lowther said parents were free to keep their children home from school, but the absence would count as unexcused.

Cassidy said she had no reservations about sending her daughter back to school Thursday. Lisa Wennerlund, a pharmacist with one child in middle school and two in elementary, did the same.

She said her two younger kids were barely aware of the situation, and her older daughter was not rattled by it.

"I think the teachers and staff did everything normal," Wennerlund said. "They just locked the doors and checked IDs. They could do that all the time and it would be fine."